The present invention relates to magnetic resonance imaging or xe2x80x9cMRIxe2x80x9d.
MRI is widely used in medical and other arts to obtain images of a subject such as a medical patient. The patient""s body is placed within a subject-receiving space of a primary field magnet and exposed to a strong, substantially constant primary magnetic field. The atomic nuclei spin around axes aligned with the magnetic field. Powerful radio frequency xe2x80x9cRFxe2x80x9d signals are broadcast into the subject-receiving space to excite atomic nuclei within the patient""s body into a resonance state in which the spinning nuclei generate minuscule RF signals. These signals are referred to herein as magnetic resonance signals. Magnetic field gradients are applied so that the magnitude of the magnetic field varies with location inside the subject-receiving space. As a result, characteristics of the magnetic resonance signals from different locations within the region, such as the frequency and phase of the signals, can be made to vary in a predictable manner, depending upon position within the region. Thus, the magnetic resonance signals are xe2x80x9cspatially encodedxe2x80x9d so that it is possible to distinguish between signals from different parts of the region. After repeating this procedure with various different gradients, it is possible to derive a map showing the intensity or other characteristics of the magnetic resonance signals versus position within the excited region. Because these characteristics vary with concentration of different chemical substances and other characteristics of the tissue within the subject""s body, different tissues provide different magnetic resonance signal characteristics. When the map of the magnetic resonance signal characteristics is displayed in a visual format, such as on screen or on a printed image, the map forms a visible picture of structures within the patient""s body.
MRI provides unique imaging capabilities which are not attainable from any other imaging method. For example, MRI can provide vivid, detailed images of soft tissues, including abnormalities such as tumors, and other structures which cannot be seen readily in X-ray images. Moreover, MRI operates without exposing the patient or the physician to ionizing radiation such as X-rays. For these and other reasons, MRI is widely utilized in medicine.
Some of the primary field magnets utilized heretofore have imposed severe physical constraints on the patient and on medical personnel attending to the patient during the MRI procedure. For example, conventional solenoidal primary field magnets use a series of circular superconducting coils spaced apart from one another along an axis. These magnets provide a small, tubular subject-receiving space enclosed within the solenoids. A patient to be imaged must slide into the tubular space. The experience may be highly claustrophobic. Some obese or pregnant patients often cannot fit inside the patient-receiving space. Moreover, it is essentially impossible for a physician or technician to reach those regions of the patient disposed inside the subject-receiving space.
Attempts have been made heretofore to create xe2x80x9copenxe2x80x9d MRI primary field magnets using ferromagnetic frames. Although these designs provide somewhat better access to the patient for certain diagnostic scanning, and a somewhat less claustrophobic experience for the patient, they are less than optimal for surgical intervention or physician/technician-assisted diagnostic procedures. For example, these designs provide limited access of physicians, surgeons, or technicians, to the patient. Additionally, the designs have difficulty providing a highly uniform field with pole dimensions desirable for surgery or diagnostic purposes.
As described, for example, in commonly assigned U.S. Pat. No. 4,707,663, other primary field magnets utilize ferromagnetic frames to route and concentrate magnetic flux into the patient-receiving space. Primary field magnets using such a ferromagnetic frame can employ permanent magnets, resistive electromagnetic coils, or superconducting coils having a relatively low number of ampere-turns while still providing a high field strength in the patient-receiving space. Moreover, such magnet assemblies can provide excellent field uniformity. Ferromagnetic frame magnets in accordance with the ""663 patent also provide a less claustrophobic, more accessible subject-receiving space.
As disclosed in co-pending, commonly assigned U.S. patent application Ser. No. 07/952,810 filed Sep. 28, 1992, now U.S. Pat. No. 5,754,085 the disclosure of which is hereby incorporated by reference herein, a ferromagnetic magnetic frame may include a pair of plate-like pole supports spaced apart from one another and supported above one another by a set of columns. In preferred magnets according to the ""085 patent, the frame defines a polar axis passing through the space between the pole supports. Preferably, ferromagnetic poles project from the pole supports adjacent the polar axis, so that the poles define a subject-receiving space at a medial plane, midway between the plates. The columns have unique shapes such that, in preferred embodiments, the columns flare outwardly in the radial direction, away from the polar axis adjacent the medial plane. The dimensions of each column in the circumferential direction, around the polar axis desirably taper so that the circumferential dimension of each column is at a minimum in a region adjacent the medial plane. As described in further detail in the ""085 patent, magnets with ferromagnetic frames in accordance with preferred embodiments of the invention taught therein can provide a unique combination of accessibility and a large, aesthetically pleasing, and non-claustrophobic, patient-receiving space and can also provide high field strength without resort to superconducting coils. Even higher field strengths can be provided where superconducting coils are used.
Magnets according to preferred embodiments taught in the ""085 patent thus provide an elegant solution to the problems of claustrophobia, lack of access and limitations on field strength and uniformity posed by prior designs. Surgical operations and other medical procedures can be performed readily on a patient while the patient is disposed inside the patient-receiving space of preferred magnets according the ""085 patent. The ability to perform surgical operations while the patient is disposed inside the patient-receiving space allows the physician to treat the patient under direct guidance of a MRI image acquired during the procedure itself. For example, as the surgeon advances a probe into the body to treat a lesion, the surgeon can see the probe and the lesion in the MRI image.
However, even with this enhanced design, the patient still perceives the MRI procedure as involving placement of his or her body into the interior of a machine. Moreover, the physician treating the patient still perceives that he or she must stand outside of the apparatus and reach into the apparatus to gain access to the patient. Accordingly, even further improvement in primary field magnet structures for MRI apparatus would be desirable.
One aspect of the present invention provides a magnet for magnetic resonance imaging apparatus which includes a frame. The frame desirably incorporates a pair of opposed ferromagnetic pole supports spaced apart from one another and a pair of ferromagnetic poles connected to the pole support. The poles project from the pole supports toward one another along a polar axis. The poles have distal ends remote from the pole supports. The distal ends confront one another and are spaced apart from one another by a gap distance so as to define a subject-receiving gap between the poles. The frame further includes one or more connecting elements extending between the pole supports. The connecting elements are spaced apart from the poles in a direction or directions transverse to the polar axis. The magnet further includes a source of magnetic flux adapted to direct flux through the frame so that the flux passes between the distal ends of the poles through the gap and returns through the pole supports and the connecting elements.
Most preferably, the magnet defines a working space alongside of the poles, between the pole supports and between the poles and the connecting elements sufficient to accommodate one or more adult human attendants. Thus, an attendant can be positioned inside the working space, within the magnet itself and can have access to a patient disposed in the gap between the poles. The working space desirably is about six feet or more high and about two feet or more wide, so that the attendant can work in a standing position. Most preferably, the working space extends entirely around the poles, and is unobstructed by any feature of the magnet itself. The magnet desirably includes a plurality of enclosing structures including walls, a floor and a ceiling which cooperatively define a room. The poles extend into the room, but the remainder of the frame desirably is at or outside the exterior of the room. For example, where the pole supports are spaced vertically apart from one another and the polar axis extends vertically, the poles project into the room from the floor and ceiling. Thus, the patient experiences entry into the MRI magnet as entry into a normal room with some structures extending from the floor and ceiling. Stated another way, the elements such as the connecting elements and pole supports are so far away from the patient that they do not create any feeling of claustrophobia. Because the physician or other attendant is inside the room and inside the space enclosed by the pole supports and connecting elements, these elements do not impede access by the physician or other attendant to the patient at all.
The connecting elements may be in the form of plates constituting one or more walls of the room as well as providing the pole supports which may be formed as further plates constituting the floor and ceiling of the room. The enclosing structure may further include concealment structure which conceals those parts of the frame constituting the walls from view from within the room. For example, the interior surfaces of the plates may be covered with conventional wall, floor and ceiling coverings. This contributes to the patient""s belief that he or she is inside a normal room. According to further aspects of the invention, the concealment structure may include surface decoration on the surfaces bounding the room as, for example, on the walls, ceiling or floor. The magnet structure may also include pole covers covering the poles and associated structure, and the surface decoration may be provided on the pole covers as well. The surface decoration may define an outdoor scene, such as a landscape or seascape incorporating a sky region. This enhances the open, non-claustrophobic environment provided by the magnet.
Because the pole supports and connecting elements are disposed outside of the area occupied by the patient and attendant, these elements can be of essentially unlimited size. Essentially any amount of ferromagnetic material can be used to provide a low reluctance flux return path and to perform uniform distribution of flux passing to the poles. Magnets in accordance with preferred aspects of the present invention thus can provide a highly concentrated, strong magnetic field in the subject-receiving gap. Magnets according to this aspect of the invention can utilize permanent magnets, super-conducting coil or, resistive electromagnetic coils as the source of electromagnetic flux. In a particularly preferred arrangement, a coil such as a resistive electromagnetic coil encircles each pole. In systems intended to provide optimum access to the patient, for surgery or other procedures requiring considerable interaction with the patient, the coils may be disposed adjacent to the pole supports. In this configuration, the working space extends around the poles between the coils. Where the polar axis extends vertically, the working space desirably extends above one coil and below the other coil. For example, one coil may be disposed beneath the floor of the room whereas the other coil may be disposed above the ceiling.
Magnets according to a further aspect of the invention are arranged to provide optimum field uniformity. In magnets according to this aspect of the invention, the coils may extend in regions of the poles adjacent the pole tips. Thus, the coil associated with each pole may extend to within about 6 inches (15 cm) of the pole tip, and desirably to within about 3 inches (7.5 cm) of the pole tip. The coil associated with each pole may be provided as a relatively narrow toroidal solenoid in the vicinity of the pole tip or, preferably, as an elongated cylindrical solenoid surrounding the pole, the solenoid having a tip end in the vicinity of the pole tip and having a support end in the vicinity of the pole support. In these systems, the working space extends around the outside of the coil. To maintain good access to the patient, the coil desirably has a radial thickness of about 6 inches or less. Magnets according to this aspect of the invention are especially well suited to use in diagnostic procedures, including imaging without other intervention during imaging and also including interventional MRI procedures such as procedures involving the use of MRI contrast media or intrabody probes during imaging. The access to the patient provided by the magnets in accordance with this aspect of the invention also allow them to be used for surgery or other procedures requiring considerable interaction with the patient.
The gap distance between the distal ends of the pole preferably is about two feet or more. The distal ends of the poles may be either circular or non-circular. In systems intended to provide optimum access to the patient, and which employ poles with circular distal ends, the ratio between the diameter of each pole distal end and the gap distance between the poles is desirably about 2 to 1 or less. In systems designed for such optimum access where the distal ends of the poles are non-circular, the ratio between the longest dimension of each pole surface and the gap distance is desirably about 2 to 1 or less, and the ratio of the shortest dimension of each pole surface to the gap distance desirably is about 1.5:1 or less.
The magnet desirably incorporates features to further enhance field uniformity in the patient-receiving gap. Where coils are employed as the source of magnetic flux, each coil encircles the associated pole. Also, the magnet desirably includes shimming features such as shim rings, slots or other elements defining magnetic flux paths having different reluctances at different distances from the polar axis. To further promote field uniformity, each pole may include a pole tip defining a distal end of the pole and a pole stem extending from the proximal end of the pole to the pole tip. The flux source is arranged to direct the flux in a forward direction through each pole. The magnet may include stem bucking magnets surrounding the pole stem. The stem bucking magnets desirably provide flux directed in a reversed direction opposite to the forward direction. This tends to minimize leakage of flux from the pole stems to the connecting elements. The relatively large spacing between the poles and the connecting elements in radial directions transverse to the polar axis helps to minimize flux leakage from the poles, so that a very large portion of the flux tends to pass between the poles. This further promotes flux uniformity and a strong field in the subject-receiving gap.
In a particularly preferred arrangement, the vertical connecting elements are disposed at least about 7 feet from the polar axis. Thus, a typical human patient can be positioned with the long axis of his or her body extending in any desired radial direction and with any portion of his or her body at the polar axis. For example, if the patient""s head is positioned at the polar axis, as where procedures or imaging are to be performed on the head, the patient""s feet can point in any direction. In one arrangement, the connecting elements include a pair of connecting elements such as a pair of opposed, heavy, plate-like walls disposed at least about 14 feet apart from one another and defining two opposite ends of a room. In other embodiments, the polar axis may extend horizontally, and the pole supports may extend along walls of the room defined by the magnet frame.
In yet another aspect of the present invention, there is provided a diagnostic facility comprising a magnet, such as the magnets described above and magnetic resonance imaging apparatus utilizing the magnetic field applied by the magnet to provide an image of a patient received in the patient-receiving gap. Preferably, this facility includes one or more devices for performing or assisting in diagnostic procedures on a patient disposed in the patient-receiving gap. These devices are accessible within the working space. Preferably, the facility includes a staging area that includes a plurality of patient-carrying devices for carrying patients from the staging area to the patient-receiving gap and for supporting patients during diagnostic procedures. The patient-carrying devices may be pre-positioned and arranged within the staging area to provide for substantially continuous usage of the magnet for performing substantially continuous diagnostic procedures. Most preferably, one of the plurality of patient-carrying devices is positioned within the patient-receiving gap of the magnet, while one or more additional patient-carrying devices are outside of the patient-receiving gap. In a particularly preferred arrangement, the facility includes a plurality of preparation rooms and an infeed passageway communicating with the preparation rooms, so that patients can be positioned on the patient-carrying devices in the preparation rooms and can be carried on such devices through the infeed passageway into the patient-receiving gap of the magnet. Most preferably, the infeed passageway extends to the patient-receiving gap of the magnet from one side of the gap, and the facility includes an outfeed passageway extending from the other side of the patient-receiving gap. The outfeed passageway desirably leads back to the preparation rooms. In this arrangement, a succession of patients can be pre-positioned on the patient-carrying devices, and can be conveyed in sequence through the infeed passageway, into the magnet for imaging and then out of the magnet after imaging and back to the preparation rooms through the outfeed passageway.
A further aspect of the invention provides methods of magnetic resonance imaging wherein a succession of patients is conveyed into and out of a magnet in the manner discussed above, each patient being imaged while that patient is positioned in the patient-receiving gap of the magnet.
In yet another aspect of the present invention, there is provided a method of magnetic resonance for diagnostic purposes including the steps of positioning a patient within a gap defined by a frame of a magnet, positioning a technician within a working space within the frame of the magnet adjacent the gap, obtaining magnetic resonance data from the patient using a magnetic field applied by the magnet. Preferably, this method includes the step of having the technician in the working space assist the patient during one or more of the following phases of the procedure: positioning the patient within the gap; performance of diagnostic procedures while the patient is within the gap; and removal of the patient from the gap. The step of obtaining magnetic resonance data optionally may be performed so as to generate a magnetic resonance image of the patient.
A further aspect of the invention provides improved resistive coils for magnetic resonance imaging static field magnets. A coil according to this aspect of the invention includes a plurality of spiral windings extending around an axis. Each winding has an inner end adjacent the axis, an outer end remote from the axis and a conductor extending in multi-turn spiral between the inner end and the outer end. The windings include one or more outward windings and one or more inward windings. The turns of each outward winding are arranged such that a point moving along the turns in a first direction of rotation about the axis moves from the inner end towards the outer end. The turns of each inward winding are arranged so that a point moving along the turns in the first direction of rotation about the axis moves from the inner end towards the outer end. The windings are stacked one above the other along the axis and are electrically connected in series with one another. The electrical connections between the windings most preferably include one or more interior connections, the inner end of one of said inward windings being connected to the inner end of one of said outward windings at each such interior connection. The connections typically also include one or more exterior connections, the outer end of one of the outward windings being connected to the outer end of one of the inward windings at each such exterior connection. Most typically, the inward windings and the outward windings are arranged in alternating sequence along the axis. The interior connections and the exterior connections are also arranged in alternating sequence along said axis. As further discussed below, such a coil will provide essentially the same magnetic performance as a solenoid wound with an equal number of turns. However, the coil in accordance with this aspect of the invention can be fabricated more readily than a conventional helical coil having multiple helical layer nested within one another, particularly where the conductor is a relatively stiff element such as a metallic bar having cross-sectional dimensions of about 5 mm or more.
Preferably, the conductors of the windings are tubular so that each said winding has a bore extending through the conductor between its inner end and its outer end. The coil may further include a plurality of coolant ports communicating with the bores of the windings. The ports serve as coolant inlets and coolant outlets for passing coolant through said windings. The coolant ports desirably are arranged so that the bore of each winding is connected in a fluid flow path between a coolant inlet and a coolant outlet, but so that each fluid flow path extends through less than all of said windings. Thus, different windings are connected in different fluid flow paths. Each fluid flow path provides flow resistance far less than that which would be expected in a single flow path extending through the entire coil. This greatly facilitates coil cooling, and minimizes heat transfer to the pole and to other surrounding structures.
Further aspects of the invention include magnets for resonance imaging including a pair of coils as discussed above and a ferromagnetic frame including a pair of ferromagnetic poles projecting along a polar axis defining a patient-receiving gap therebetween, the poles having proximal ends remote from the gap and tips bounding the gap. The frame further includes a ferromagnetic flux return structure extending between poles.
Yet another aspect of the invention provides methods of fabricating a coil for a magnetic resonance imaging magnet. Methods according to this aspect of the invention desirably include the steps of a plurality of inward and outward spiral windings as discussed above, the windings being separate from one another; superposing the spiral windings one above the other so that said spiral windings are substantially coaxial with one another; and electrically connecting the windings in series with one another. The connecting step desirably includes the step of forming one or more interior connections and exterior connections as discussed above. Preferably, the conductors constituting the windings are tubular. The step of electrically connecting the windings in series with one another includes the step of joining the tubular windings to one another so as to form a continuous fluid path between windings at at least some of said connections.
Yet another aspect of the invention provides a magnet for magnetic resonance imaging having a ferromagnetic frame defining a patient-receiving space and a source of magnetic flux in magnetic circuit with said frame so that flux produced by said source will pass through said patient-receiving space and through said frame; and a temperature suppresser or means for suppressing temperature changes in the frame during operation. Most preferably, the temperature suppresses thermal insulation covering at least a part of said frame. This aspect of the invention incorporates the realization that changes in the magnetic properties of the frame due to changes in temperature of the frame can cause changes in magnetic fields in the patient-receiving space. By suppressing these temperature-induced changes, the magnet according to this aspect of the invention provides enhanced field stability and uniformity.
These and other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings.